In a plasma processing apparatus, it is very important to correctly measure temperature of a substrate to be processed, e.g., a semiconductor wafer or a substrate for a liquid display device, in order to accurately control shape and property of a film, a hole or the like formed thereon by various processes such as a film formation, etching and the like. Therefore, various methods such as a measurement method using a resistance thermometer, a fluorescent thermometer measuring a temperature of a backside of a substrate or the like, have been used to measure temperature thereof.
Recently, there has been proposed a temperature measurement technique using a low coherence interferometer capable of directly measuring temperature of a substrate, which has been difficult by using a conventional temperature measurement method. Further, there has been suggested a temperature measurement technology using such a low coherence interferometer, which can measure temperatures at plural measuring points at the same time (see, e.g., Japanese Patent Application Publication No. 2006-112826).
In the technology, a light beam from a light source is divided into a measurement beam for measuring a temperature and a reference beam by a first splitter and the measurement beam is further divided into n measurement beams by a second splitter. The n measurement beams are irradiated onto n measuring points and the reference beam is irradiated on a reference beam reflector. Then, an interference wave of reflected n measurement beams from the target and a reflected reference beam from the reference beam reflector is measured by a light receiving unit. With the technology, it is possible to measure temperatures at plural measuring points at one time with a simple configuration.
When a temperature of a substrate which is being processed by a plasma processing apparatus is measured by a temperature measuring apparatus by using the above low coherence interferometer, the substrate is mounted on a mounting table in a vacuum chamber under a vacuum atmosphere. Meanwhile, a collimator disposed at an outlet of an optical fiber for guiding a measurement beam is generally fixed to an outside of a base plate in a vacuum processing chamber. Herein, the outside of a base plate in the vacuum processing chamber normally set to be maintained at an atmospheric pressure, for convenience of maintainability such as an optical axis alignment or the like.
In the vacuum processing chamber, the mounting table for mounting thereon the substrate includes an electrostatic chuck for attracting and holding a substrate and an RF plate to which a high frequency power is applied, and the vacuum processing chamber is configured to be partitioned off into a space under the vacuum atmosphere and another space under the normal pressure atmosphere by them. Below the mounting table, in order to sufficiently insulate the RF plate and the base plate or install a driving unit of a pusher pin for loading and unloading a semiconductor wafer to and from the mounting table, there may have a space between the mounting table and the base plate.
In this configuration, the mounting table may be bent by the pressure difference between the vacuum and the atmospheric pressure, or may vibrate due to a flow of temperature-controlling cooling medium therein. Accordingly, the distance between the collimator and the substrate mounted on the mounting table can be changed, and an accurate temperature measurement cannot be achieved. Further, since there is the atmosphere in the space between the RF plate and the base plate, an optical path is affected by an air flow, thereby deteriorating measurement accuracy.
Moreover, the problems that the optical path is affected by the air flow and the measurement accuracy deteriorates are also seen when a temperature of a focus ring provided in the plasma processing apparatus is measured.